1. Field of the Invention
The present invention relates to a method for producing a heat mode type optical information-recording medium capable of recording and reproducing information by using a laser beam and the optical information-recording medium.
2. Description of the Related Art
In general, the optical information-recording medium (optical disk) capable of recording information only once by using the laser beam includes, for example, write-once type CD (so-called CD-R) and DVD-R. Such an optical information-recording medium is advantageous in that a small amount of CD's can be commercially supplied to the market quickly at a convenient price as compared with the production of conventional CD (compact disk). The demand for such an optical information-recording medium is increasing in accordance with the recent popularization of personal computers or the like.
The optical information-recording medium of the CD-R type has a representative structure comprising a recording layer composed of an organic dye, a light-reflective layer composed of a metal such as gold, and a protective layer made of a resin which are stacked in this order on a transparent disk-shaped substrate having a thickness of about 1.2 mm (see, for example, Japanese Laid-Open Patent Publication No. 6-150371).
The optical information-recording medium of the DVD-R type has a structure comprising two disk-shaped substrates (having a thickness of about 0.6 mm) which are laminated with each other with respective information-recording surfaces disposed inwardly in an opposing manner. The optical information-recording medium of this type has such a feature that a large amount of information is recorded thereon.
Information is written (recorded) on the optical information-recording medium by radiating a near infrared laser beam (laser beam usually having a wavelength of about 780 nm in the case of CD-R or a wavelength of about 635 nm in the case of DVD-R). A portion of the dye recording layer, which is irradiated with the laser beam, absorbs the light, and its temperature is locally raised. As a result, a physical or chemical change (for example, generation of pit) takes place, and the optical characteristic is changed. Thus, the information is recorded.
On the other hand, information is also read (reproduced) by radiating a laser beam usually having the same wavelength as that of the recording laser beam. The information is reproduced by detecting the difference in reflectance between a portion (recorded portion based on the generation of pit) which has subjected to the change in optical characteristic of the dye recording layer and a portion (non-recorded portion) which is not subjected to the change.
For example, a lot number, a manufacturer, and a disk type indication, which indicate the manufacturing history of an optical disk, are printed on a transparent portion disposed on the inner circumferential side of the substrate in some cases. Although such printing forms do not affect the recording operation on the optical disk itself, the quality of their appearance is considered to be important.
As shown in FIG. 27, when a recording layer based on an organic dye is formed on a substrate 1000 of an information-recording medium as described above, a dye solution 1002 is applied onto the substrate 1000 via a nozzle 1004 while rotating the substrate 1000. Therefore, the dye solution 1002, which falls onto the substrate 1000, is occasionally scattered in all directions. It is feared that any defective appearance is brought about at a transparent portion 1006 disposed on the inner circumferential side.
In view of the above, as shown by a two-dot chain line in FIG. 27, a method is conceived, in which a mask 1008 is installed on a portion to which it is not intended to allow any droplet to adhere. However, every time when the application process is carried out, the surface of the mask 1008 is stained by the droplets of the dye solution 1002. When the stain is dried, it is scattered as dust. For this reason, it is feared that a new problem arises in that such a process may cause any defect.
In the case of the heat mode type optical information-recording medium as described above, the tracking servo for the laser beam is performed on the basis of the push-pull signal.
The push-pull signal is dealt with such that the signal intensity obtained when the groove is deviated by 0.1 .mu.m is normalized with the signal intensity of the reflectance, for example, in the case of CD-R. This procedure is performed as follows. That is, the amplitude of the push-pull signal, which is obtained when the tracking is deviated, is measured to obtain the value by means of calculation.
The push-pull signal, which is used for the tracking servo, is approximately at the zero level when the scanning is regularly performed along one groove by using the laser beam. When the laser beam is deviated from one groove toward the outer circumferential side or the inner circumferential side, the push-pull signal is at a level corresponding to an amount of the deviation. Whether the laser beam is deviated toward the outer circumferential side or the inner circumferential side can be known from the polarity of the signal.
Therefore, when the change in level of the push-pull signal is monitored, then it is possible to perform the tracking servo for one groove by using the laser beam, and it is possible to make access to a desired track address as well.
The conventional optical information-recording medium is manufactured as follows. That is, when the recording layer based on the organic dye is formed on the substrate, the dye solution is applied onto the substrate while rotating the substrate. Especially, the dye solution is applied while rotating the substrate at a constant number of revolutions irrelevant to whether the application is performed for the outer circumferential side or the inner circumferential side of the substrate.
For this reason, the film thickness of the applied dye becomes thick on the inner circumferential side of the substrate. Therefore, for example, the signal intensity of the push-pull signal for the tracking servo is decreased in some cases, and the dispersion is increased as well for the level of the push-pull signal concerning the radial direction of the substrate. In such a case, the tracking deviation of the laser beam occurs highly possibly.
The film thickness, the push-pull, and the reflectance not necessarily behave in a constant manner. However, in general, the following trade-off relationship holds. That is, when the push-pull is increased due to the change in film thickness, the reflectance is decreased.
On the other hand, the increase in signal intensity of the push-pull signal is based on the excessive decrease in reflectance. In this case, an inconvenience arises in that the reading error tends to occur due to the excessive decrease in amplitude of the reproduced signal.
The cost of the dye solution occupies about 30% of the cost of the material for the optical disk. Therefore, in order to reduce the production cost of the optical disk, it is necessary to decrease the amount of application of the dye solution onto the substrate as less as possible.
If the amount of application of the dye solution is decreased, the following problems arise. That is, the shape of the inner circumferential applied portion is disturbed, the distribution of the film thickness of the dye (in the circumferential direction) is deteriorated, and the non-applied portion appears.
The following techniques are available in order to decrease the amount of application of the dye solution. However, each of the techniques involves any problem.
(1) The discharge pressure is lowered. This technique involves the following problem. That is, even if the dye solution is applied to the substrate, then the dye solution flows in a so-called ball form, it rolls on the substrate, and it falls from the substrate. As a result, it is impossible to apply the dye solution uniformly onto the entire surface of the substrate. In this case, problems arise in that the non-applied portion appears, and the film thickness is not uniform.
(2) The application is performed in a shortened period of time. This technique involves the following problem. That is, the dye solution is not distributed over the entire substrate, and problems arise in that the non-applied portion appears, and the film thickness is not uniform.
(3) The nozzle is stopped for a shortened period of time at the inner circumference of the substrate. This technique involves the following problem. That is, the shape of the applied portion at the innermost circumference is disturbed, and it does not form a perfect circle. Also in this case, the film thickness is not uniform.
(4) The nozzle is moved at a fast speed. This technique involves the following problem. That is, even if the dye solution is applied to the substrate, then the dye solution flows in a so-called ball form, it rolls on the substrate, and it falls from the substrate. As a result, it is impossible to apply the dye solution uniformly over the entire surface of the substrate. In this case, problems arise in that the non-applied portion appears, and the film thickness is not uniform.